The oil sands in Northern Alberta constitute one of the largest hydrocarbon reserves in the world. Oil sands are a combination of bitumen, quartz sand, clay, water and trace minerals. Bitumen can be recovered from oil sands using two main methods: open-pit mining and in situ drilling. Approximately 20% of the oil sands lie close enough to the earth's surface to be mined.
The key characteristic of Alberta oil sand that makes bitumen economically recoverable is that the sand grains are hydrophilic and encapsulated by a water film which is then covered by bitumen. The water film prevents the bitumen to be in direct contact with the sand and, thus, by slurrying mined oil sand with heated water, the bitumen is allowed to be liberated from the sand grains and move to the aqueous phase. A primary separation vessel (PSV) is normally used for bitumen separation and bitumen froth production.
The PSV product, or primary bitumen froth, is a mixture of bitumen, water, and solids. The target composition of this froth product is ≥60 wt % in bitumen, ≤30 wt % in water, and ≤10 wt % in solids. To enable downstream upgrading, the PSV froth must first be cleaned in a froth treatment process to reduce the water and solids contents to desirable levels. However, when the quality of the PSV froth becomes poorer and, in particular, when the froth solids content becomes high (i.e., greater than 10 wt %), it will negatively impact the froth treatment process and the quality of the final product of the froth treatment process.
Several different water-based bitumen extraction processes have been developed throughout the years. One such extraction process is commonly referred to in the industry as the “hot water process”. In general terms, the hot water process involves feeding the mined oil sand into a rotating tumbler where it is mixed for a prescribed retention time (generally in the range of 2 to 4 minutes) with hot water (approximately 80-90° C.), steam, caustic (e.g., sodium hydroxide) and naturally entrained air to yield a slurry that has a temperature typically around 80° C. The bitumen matrix is heated and becomes less viscous. Chunks of oil sand are ablated or disintegrated. The released sand grains and separated bitumen flecks are dispersed in the water. To some extent bitumen flecks coalesce and grow in size. They may contact air bubbles and coat them to become aerated bitumen. The term used to describe this overall process in the tumbler is “conditioning”. The conditioned oil sand slurry is then subjected to gravity separation, generally in a PSV, to produce a bitumen froth product.
Another extraction process, which is disclosed in Canadian Patent No. 2,029,795 and U.S. Pat. No. 5,039,227, involves the use of a pipeline to condition oil sand slurry. In this process, heated water (typically at 95° C.) is mixed with the dry as-mined oil sand at the mine site in predetermined portions using a device known as a “cyclofeeder”, to form an aerated slurry having a temperature in the range of 40-70° C., preferably about 50° C. The slurry is then pumped to the extraction plant through several kilometres of pipeline, where conditioning (i.e., lump digestion, bitumen liberation, coalescence and aeration) occurs. Once again, the conditioned oil sand slurry is then subjected to gravity separation, generally in a PSV, to produce a bitumen froth product.
In an attempt to reduce the thermal energy requirement per tonne of oil sand, a low energy extraction process for extracting bitumen from oil sand was developed, which is disclosed in Canadian Patent No. 2,217,623 and U.S. Pat. No. 6,007,708. This process involves mixing the mined oil sand with water in predetermined proportions in a mix box located near the mine site to produce a slurry containing entrained air and having a controlled density in the range of 1.4 to 1.65 g/cc and preferably a temperature in the range 20-40° C. The slurry is then pumped through a pipeline to condition the slurry. Once again, the conditioned oil sand slurry is then subjected to gravity separation, generally in a PSV, to produce a bitumen froth product.
Another bitumen extraction process uses a slurry preparation unit as described in Canadian Patent No. 2,480,122. In this process, little or no rejects will be produced during slurry preparation. The slurry preparation unit comprises a series of roll crushers spread vertically throughout a portion of a slurry preparation tower. The slurry preparation tower typically uses gravity to move the oil sand through the tower. Typically, each roll crusher is made up of a number of crusher rolls spaced a set distance apart to reduce the size of large pieces of oil sand before the pieces of oil sand drop through the crusher rolls to the next roller crusher beneath or the bottom of the slurry preparation tower. Each successively lower roll crusher reduces the pieces of oil sand even smaller until the oil sand is fine enough to form a pumpable oil sand slurry. Once again, the conditioned oil sand slurry is then subjected to gravity separation, generally in a PSV, to produce a bitumen froth product.
In the existing water-based bitumen extraction processes, generally caustic (e.g., sodium hydroxide) is used as a process aid to improve the overall performance, including froth quality. Caustic helps the release of natural surfactants and affects surface properties of bitumen, sand, and clays. The use of caustic reduces the attachment of fine solid particles on bitumen surface.
Another technology that has been employed mainly for froth quality improvement involves the use of a high temperature froth underwash in a separation vessel such as a PSV (see U.S. Pat. No. 9,051,518). Froth underwash is the gentle and uniform distribution of a water layer at the vicinity of the froth-middlings interface in a PSV. Its function is to establish a favorable environment for the froth formation step by facilitating the rising of bitumen droplets and preventing solids/fines to reach the froth-middlings interface.
Despite these advancements, froth quality remains an on-going concern with many of the aforementioned extraction operations. For example, when using the low energy extraction process, the PSV froth solids content doesn't always meet the target of ≤10 wt %, and, often, the solids content is in the range of ≥12 wt % up to as high as 16%. The hot water process generally yields better results; however, still, the average froth solids content is generally around 11.0 wt % and can be up to 14% or higher.
Thus, high froth solids content remains a problem in the industry and even though significant efforts have been made over the years, reducing froth solids content is still a challenge in the industry.